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VLT Unit Telescope 1 in its Enclosure (17 June)
The VLT Sharpens its View (10 June)

VLT Astronomical Images : Index

Information from the European Southern Observatory

May 27, 1998

For immediate release

First Astronomical Images from the VLT UT1


Following a decade of intensive work on the VLT Project, astronomically useful images have been obtained with the first of the 8.2-m Unit Telescopes at the Paranal Observatory. More details about this important event are available on ESO Press Release 06/98 (May 27, 1998).

The present collection consists of images which have been taken with the UT1 at the time of the first valid observations. Together, they provide a most convincing illustration of the great potential of this new research facility. The photos are accompanied by captions that explain the details and highlight their particular significance, also in terms of telescope performance.

All photos are available in two JPEG versions: a smaller preview and a larger high-resolution one. Clicking on the icons will show the preview version.

Two other series of photos have been published earlier in connection with the VLT First Light: VLT Milestones (Set no. 1) and The Paranal Observatory in April 1998 ( Set no. 2).

These photos may be reproduced, if credit is given to the European Southern Observatory and a copy of the publication in which they appeared is sent to the ESO EPR Dept. at the address below.

Omega Centauri Tracking Test

VLT UT1 First Light Photo 03/01

VLT UT1 First Light Photo No. 1

Preview [JPEG: 328k]
High-resolution [JPEG: 1467x1698 pix; 1368k].

Omega Centauri is the most luminous globular cluster in our Galaxy. As the name indicates, it is located in the southern constellation Centaurus and is therefore observable only from the south.

The image shown here was obtained with the VLT on May 16, 1998, in red light (R band), i.e. while the mirror was still uncoated. It is a 10-minute exposure of the center of Omega Centauri and it demonstrates that the telescope is able to track continuously with a very high precision and thus is able to take full advantage of the frequent, very good atmospheric conditions at Paranal. The images of the stars are very sharp (full-width-at-half-maximum (FWHM) = 0.43 arcsec) and are perfectly round, everywhere in the field. This indicates that the tracking was accurate to better than 0.001 arcsec/sec during this observation.

At a distance of about 17,000 light years, this cluster is barely visible to the naked eye as a very faint and small cloud. When Omega Centauri is observed through a telescope, even a small one, it looks like a huge swarm of numerous stars, bound together by their mutual gravitational attraction.

Most globular clusters in our Galaxy have masses of the order of 100,000 times that of the Sun. With a total mass equal to about 5 million solar masses, Omega Centauri is by far the most massive of its kind in our Galaxy.

The Quadruple Clover Leaf Quasar

VLT UT1 First Light Photo 03/02

VLT UT1 First Light Photo No. 2

Preview [JPEG: 71k]
High-resolution [JPEG: 1417x1435 pix; 424k].

The phenomenon of gravitational lensing occurs when light rays are deflected by the gravitational influence of an intervening, massive object e.g. a heavy galaxy, along the light path. Gravitational lensing was originally studied as a curiosity ("cosmic mirages"), but is now known to be a powerful tool for mapping the mass distribution of the lensing objects. In particular, it may provide information on the mysterious dark matter in the Universe. Under favourable circumstances, it may also be used for the determination of the Hubble Constant, that is related to the age of the universe.

The Clover Leaf quasar (QSO 1413+117) is located in the constellation Bootes. It is a text-book case of a distant quasar, (at redshift 2.6, i.e. the look back time is 80% of the age of the Universe), whose single image has been split into four different components by the gravitational action of a very faint, but massive, intervening galaxy. This galaxy, however, is not observed in visible light and hardly detected in the infrared.

All four components exhibit nearly identical spectra, with rare features leaving no doubt on their lensing origin. The compactness of this object - the largest distance between two components is only 1.35 arcsec - makes it very difficult to observe in detail, especially with ground-based telescopes. Its study calls for very good instrumental and atmospheric conditions.

This is a 3D-representation of a 2-minute VLT image of the Clover Leaf quasar, obtained on May 16, 1998, in red light (R-band) during a period of excellent seeing (0.32 arcsec), as measured with a seeing monitor at the top of Paranal. The recorded, angular resolution of this image is just 0.38 arcsec and demonstrates the near-perfect optical quality of the telescope. It is certainly one of the best ground-based images of this object ever obtained.

The Central Region of Globular Cluster Messier 4

VLT UT1 First Light Photo 03/03

VLT UT1 First Light Photo No. 3

Preview [JPEG: 176k]
High-resolution [JPEG: 1417x1661 pix; 752k].

The globular cluster Messier 4 (M4 for short) is located about 6,000 light-years away. On the sky, it is seen in the southern constellation Scorpius, It is an ensemble of about 500,000 stars, held together by their mutual gravitational pull. There are about 150 such clusters in our Galaxy, and M4 is the one closest to us.

This colour picture has been obtained by combining three images taken through three different filters at the wavelengths of blue, green, and red light. They were obtained during the night of May 22, 1998. In this way, the stars are seen in their true colors, ranging from blue for very hot stars (about 10,000 degrees) to red for the cooler ones (about 4,000 degrees).

With an exposure time of only 2 minutes, the VLT has been able to detect in the blue light stars as faint as magnitude 24. This corresponds to 15 million times dimmer than the faintest stars visible to the naked eye. This was achieved even though the image has a fairly high background, being taken with the Moon above the horizon (3 days before New Moon, with 18% illumination). The large mirror surface of the VLT UT1 (53 m2) and its ability to produce very sharp images (measured as 0.53 arcsec on these images), unequalled by any ground-based telescope, ensures that faint objects may be observed extremely efficiently, especially under the good conditions that prevailed during this observation.

Note that, because of the limitations in the printing process, these faint stars cannot easily be seen in this glossy reproduction.

Fine Structure in the Butterfly Nebula

VLT UT1 First Light Photo 03/04

VLT UT1 First Light Photo No. 4

Preview [JPEG: 168k]
High-resolution [JPEG: 1417x1778 pix; 1504k].

This splendid colour image of a famous southern Planetary Nebula, the Butterfly (NGC 6302), was obtained by combining blue, yellow and red images obtained on May 22, 1998, with 10 minute exposures and an image quality better than 0.6 arcseconds.

Towards the end of their life, some massive stars expand to giant dimensions. They shed most of the hydrogen in their outer layers as a strong "stellar wind", before they contract towards a final compact stage as "white dwarfs".

After this ejection process, the star remains thousands of times brighter and also much hotter than the Sun during a few thousand years. Its strong ultraviolet radiation has the effect of ionizing the previously ejected gas, which then shines before it disperses into interstellar space. The resulting nebulae (traditionally referred to as Planetary Nebulae, because of their resemblance to a planet in a small telescope) often exhibit very complex morphologies.

The Butterfly Nebula belongs to the class of bipolar nebulae, as this picture clearly illustrates. A dark, dusty and disc-like structure - seen edge-on in this image - obscures the central star from our view. However, its strong radiation escapes perpendicular to the disk and heats and illuminates the material deposited there by the stellar wind.

The origin of the dark disk may be due to the central star being a member of a double star system. This has been shown to be the case in some other bipolar nebulae in which, contrary to the Butterfly Nebula, there is a direct view towards the star.

High-Velocity Ejecta in Eta Carinae

VLT UT1 First Light Photo 03/05

VLT UT1 First Light Photo No. 5

Preview [JPEG: 240k]
High-resolution [JPEG: 1417x1709 pix; 1784k].

Around 1841, Eta Carinae became one of the brightest stars in the sky when it underwent a giant outburst. This 10-second image was obtained in red light on May 22, 1998, with the coated 8.2-m VLT mirror. It beautifully demonstrates the detailed structure of the material that was ejected on that occasion. While many of the individual structures that can be recognised here have been subject to previous investigations, this is the most detailed ground-based image ever obtained of this extensively studied object and its immediate surroundings.

The bright star in the center is heavily overexposed, leading to "blooming" effects in the digital detector and adding a signature of the "spiders" that hold the secondary mirror in position.

Another image was obtained with the UT1 Cassegrain Guide Probe under excellent seeing conditions (0.38 arcsec), see the insert at the bottom right corner. It shows Eta Carinae and the Homunculus nebula in great detail. The orientation of the two images is identical.

Eta Carinae is one of the heaviest stars in the Milky Way and is situated at a distance of 7,500 light-years. Its mass is estimated to be about 100 times that of the Sun. Such heavy stars burn their hydrogen at an exceptionally high rate, so that they become intrinsically very bright, about a million times more luminous than the Sun. A consequence is that they evolve much faster and explode as supernovae only a few million years after their birth.

Eta Carinae has now entered the final stage of its life and will most probably become a supernova some time during the next 100,000 years. In the present stage, it is highly unstable. It undergoes giant outbursts at least once every one thousand years during which it sheds an enormous amount of mass.

This fine image shows the remnants of Eta Carinae's most recent outburst that took place around 1841, that is, simultaneously with the dramatic brightening of the object. Moreover, at the left-hand side, several faint isolated features (known as the "East condensations") show up in great detail (see the insert at bottom left corner). They may have been ejected in the 15th century, some 400 years earlier, during an outburst similar to the one that ocurred in 1841.

The image also includes a linear, jet-like feature (insert upper right corner) that displays blobs of gas moving with velocities exceeding 1000 km/s. A bow shock at the head of the jet-like feature is clearly visible.

The Dust Band in Centaurus A

VLT UT1 First Light Photo 03/06

VLT UT1 First Light Photo No. 6

Preview [JPEG: 256k]
High-resolution [JPEG: 1417x1791 pix; 2040k].

Centaurus A is the closest active elliptical galaxy and one of the strongest radio sources in the sky. This image shows part of the dust lane that obscures the central regions of the galaxy. This complex structure is believed to be the result of the recent collision between the old elliptical galaxy and a dwarf, gas-rich galaxy. Intense star formation is taking place within the violently stirred gas during the merging event.

This image was taken with the Test Camera of the VLT UT1 telescope on May 22, 1998, during a short, 10 sec exposure through a red filtre to demonstrate the great light collecting power of the 53 m2 mirror of the VLT UT1. It shows a great deal of fine details. The image quality is about 0.49 arcsec.

The insert shows a complete view of Centaurus A taken with another telescope. The brightest stars are foreground objects located within our own galaxy, but clusters of recently formed stars are visible at the edge of the dust lane.

With powerful infra-red detectors to be mounted on the VLT later this year, astronomers will soon be able to probe deep into the dust lane, infrared light being less absorbed by dust than red light.

The Energetic Jet in Messier 87

VLT UT1 First Light Photo 03/07

VLT UT1 First Light Photo No. 7

Preview [JPEG: 464k]
High-resolution [JPEG: 1698x1437 pix; 1680k].

Messier 87 (NGC 4486) is a giant elliptical galaxy that harbours an active nucleus in its centre. The central black hole is fed by a small gas disk, and it powers a highly collimated, energetic jet that penetrates the inner part of the galaxy.

As a typical elliptical galaxy, it primarily contains old, fairly cold stars - they cause the reddish colour of the galaxy in this colour image. The powerful jet, however, is extremely energetic and emits much of its energy in the blue and ultraviolet parts of the spectrum. The jet, therefore, appears bright blue in comparison.

This image is a colour composite of three images taken in ultraviolet, blue, and visible light (U, B and V filtres) during the night of May 25 - 26, 1998. In reality, the bright blue colour of the jet corresponds to ultraviolet radiation.

The atmospheric conditions were less than optimal during this exposure.

Total Optical Control

VLT UT1 First Light Photo 03/08

VLT UT1 First Light Photo No. 8

Preview [JPEG: 112k]
High-resolution [JPEG: 1417x1671 pix; 472k].

The VLT active optics system fully controls the primary and secondary mirrors. In this composite, images of stars are shown that were obtained during tests in which the control system forced the optical elements to produce three different image aberrations, namely triangular, round (defocus) and linear (astigmatism). This is done by applying different forces to each of the 150 individual active actuators on which the 8.2-m main mirror rests and to the position of the secondary mirror. In the case of the triangular aberration, the mirror was made to resemble Napoleon's hat. It is worth noting that the deviation of the mirror from its optimum shape is only 0.015 millimetres.

The great power of this system is demonstrated by the fact that the resulting stellar images can take on (nearly) any desired form. The optical system is also consistently brought back to its optimal form, producing the sharp images of a real star, shown in the lower row.

The control of the two mirrors is such that no significant aberrations remain after the corrections are applied. The accuracy of the control of shape of the primary mirror results in an average error of order 0.00005 millimetres. The telescope is only limited by the Earth's atmosphere. In space, the optical quality of the mirrors under active control could be diffraction limited.

Image Quality of the VLT

VLT UT1 First Light Photo 03/09

VLT UT1 First Light Photo No. 9

Preview [JPEG: 120k]
High-resolution [JPEG: 1417x2012 pix; 520k].

Superb image quality is the prime requirement for the VLT. The VLT should take full advantage of the exceptionally good ``seeing" conditions of the Paranal site, i.e. moments of a particularly stable atmosphere above the site, with a minimum of air turbulence.

In this diagram, the measured image quality of the VLT UT1 astronomical images is plotted versus the "seeing", as measured by the Seeing Monitor, a small telescope also located on top of the Paranal Mountain.

The dashed line shows the image quality requirement, as specified for the VLT at First Light. The dotted line shows the specification for the image quality, three years after First Light, when the VLT will be fully optimized. The fully drawn line represents the physical limit, when no further image distortion is added by the telescope to that introduced by the atmosphere.

The diagram demonstrates that First Light specifications have been fully achieved and, impressively, that the actual VLT performance is sometimes already within the more stringent specifications expected to be fulfilled only three years from now.

Various effects contribute to degrade the image quality of a telescope as compared to the local seeing, and must be kept to a minimum in order to achieve the best scientific results. These include imperfections in the telescope optical mirrors and in the telescope motion to compensate for Earth rotation during an exposure, as well as air turbulence generated by the telescope itself. The tight specifications shown in this figure translate into very stringent requirements concerning the quality of all optical surfaces, the active control of the 8.2-m mirror, the accuracy of the telescope motions, and, in the near future, the fast ``tip-tilt" compensations provided by the secondary mirror, and finally the thermal control of the telescope and the entire enclosure.

The only way to achieve an image quality that is "better than that of the atmopshere" is by the use of Adaptive Optics devices that compensate for the atmospheric distortions. One such device will be operative on the VLT by the year 2000, then allowing astronomers to obtain images as sharp as about 0.1 arcsec.

In this diagram, both seeing and telescope image quality are measured as the full-width-at-half-maximum (FWHM) of the light-intensity profile of a point-like source. The uncertainty of the measurements is indicated by the cross in the lower right corner.

© ESO Education & Public Relations Department
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